The overall objective of the research program is to increase our understanding of spoken language comprehension. Our specific focus is on the initial stages of language processing, during which the speech signal is analyzed with respect to those acoustic properties that specify the segmental structure of the utterance -- the sequences of consonants and vowels that define the lexical items of the language. It is known that the mapping between acoustic signal and phonetic structure is complex, in that it changes substantially as a function of numerous contextual factors. One of these is speaking rate. During the course of normal conversation the rate at which an individual speaks varies widely, and this change in rate systematically alters many of the acoustic properties that convey segmental information. The critical issue for a theory of spoken language processing is how the listener is able to comprehend the utterance, despite this variation in the speech signal. Recent research indicates the manner in which this might be accomplished: It appears that during language processing the listener appropriately, and with great precision, analyzes the segmentally-relevant acoustic information in relation to the rate at which the utterance was produced, rather than in an absolute manner. The goal of the proposed research is to elucidate the nature of such rate-dependent processing. This research constitutes an extension of our ongoing research program, and focuses on three fundamental issues: (1) Conditions of rate-dependent processing; (2) Effect of rate on the internal structure of phonetic categories; and (3) Nature of the relevant rate information. We will investigate these issues by conducting parallel studies of speech perception and speech production. Taken together, these studies will allow us to specify the way in which the processing system accommodates for the complex alterations in the acoustic fine-structure of speech that result from a change in rate. The proposed research will provide fundamental information on the initial stages of language processing. In so doing it will place important constraints on a theory of speech perception and contribute toward our knowledge of normal language comprehension. A complete understanding of the processes involved in the comprehension of spoken language is not only important in its own right, but provides a critical basis for the study of both language acquisition and language disorder.